1334
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 23, No. 12
INDUSTRIAL COATINGS FROM THE CONSUMER’S VIEWPOINT Papers presented before the Division of Paint and Varnish Chemistry at tha S2nd Meeting of the American Chemical Society, Buffalo, N. Y.,Augwt a 1 to September 4, 1981
Requirements of Airplane Coatings’ J. L. McCloud FORDMOTORCo.,DEARBORN, MICA.
The important factor at present in coatings for airHE present day increasbe taken over for this latter planes is a finish to protect and ornament metal suring i n t e r e s t in airwork. T h e e x t r e m e durafaces. Airplanes, having been designed for maximum planes h a s b r o u g h t bility and advantages of a p strength-weight ratio, are high in surface-weight ratio into prominence the finishing plication of n i t r o c e l l u l o s e so that durability can no longer be judged by apof a i r c r a f t . There are, of coatings have made their use pearance alone. Of far greater importance is the ability course, two distinct types of here the obvious one. To an of the finishing system as a whole so to protect the airplanes. The earlier ones extent the problems of the metal from such corrosion as will affect its strength. are those constructed chiefly automobile finishes, as given The greater portion of present day airplane strucof wood, covered with fabric. by Mougey and Wirshing (6), tures is of light metal alloys, and these are prone to I n this type of ship the first will hold here. corrosion of the intercrystalline type. Such corrosion consideration is damp protecProbably the a l m o s t unilowers the elongation obtained in tensile test specition of wood parts and the versal method in automobile mens and makes what is called a more brittle condidoping of the fabric. This construction is the application. tion of a baking “oil-base” latter is primarily for shrinkThe best way to evaluate protective coatings is to primer, followed by b a k i n g ing and making taut the skin; apply them to test bars of thin light metals and steels. and it is later given finishing surfacers and then nitrocelluThese coated test pieces are exposed to corroding condilose l a c q u e r s . It can be coats. The dope used is of tions, and the changes in physical properties measured readily appreciated, however, the cellulose ester type, and and plotted. Such testing methods are new to paint that in the much more limited finishing of this type is well a i r c r a f t construction, with manufacturers but are necessary to properly evaluate described by Gardner (4). generally much larger units, airplane finishes. Such comparisons, carried out in Considerable work has also long-time salt-spray corrosion tests and exposures to such a system presents diffibeen done by the Forest Prodculties. The more locrical one ucts Laboratory on wood fin- Florida roof conditions, are given. is to use a q u i c k e r - d r y i n g ishing; Dunlap (3)has given the results of some test work. Here he describes the particu- primer, and generally the lacquer is applied directly over this. lar advantage of aluminum additions to paint vehicles, par- Pyroxylin or nitrocellulose primes are used, but here, as in automotive work, the danger in their use is that the metal will ticularly asphalt paints. There is, however, a newer type of airplane construction not be sufficiently clean, and adhesion is then so low that in which is rapidly gaining popularity-namely, the metal service the finish will almost literally blow off. There is, however, the more important service to be renship. These ships were a t first mostly steel tubular frames and were fabric-covered, but they are now fast becoming dered by aircraft finishes of corrosion protection than is reall-metal sups. They may be largely constructed of alumi- ferred to above. It is connected with that low weight requirement which is so essential, and which is thereby brought num alloys, or of what might be called mixed constructionfor example, steel as well as aluminum alloys. The choice into prominence. Corrosion in metals starts from the surof these metals is governed largely by what may be called face, and, in a structure that is largely surface, it must be form; that is, if the member is large enough or strong enough, prevented even from starting. It can readily be appreciated and yet if it has enough inherent stiffness, it may be ad- how much this surface-weight ratio may be when a tube is vantageously made of steel. Strong aluminum alloys have considered. Taking, for example, a rod 0.8 inch in diameter, weight-strength ratios equal to heat-treated alloy steels, this same weight of metal if distributed as a tube of 1.32 and have excellent stiffness. With ships constructed of inches outside diameter and 1.05 inches inside diameter, these materials, the coating problems are of a different type. that is, a wall of 0.135 inch, would have the same weight and tensile strength and yet have a resisting moment toward Prevention of Corrosion bending of four and a half times a3 much as the bar, and would I n a metal ship (it being of course necessary to watch for have three times as much surface. This shows how much surface amounts to and how imporounces of weight increase) the construction is of a system of thin metal plates, tubes, and other shapes. Strength tant the problem of surface corrosion is. I n early work on and stiffness (or rigidity) both have equal importance with corrosion protection the observations have all been on a p low weight, and, since safety demands that neither of these pearance, the A. S. T. M. test work on the Havre de Grace be lost in operation of the ship, the primary object of these bridge (2) rates the paints used on the basis of appearance. coatings is to protect the metal from corrosion. What can I n a bridge the metal is relatively so much thicker that surbe termed a secondary use is to ornament the surface. The face corrosion is of much less importance. There is still experiences in the automobile field can in a large measure another reason for this surface importance-the type of corrosion in light metal alloys which is intercrystalline. 1 R e e k e d September 10, 1931.
T
v
INDUSTRIAL A N D E N ~ l N h ~ E R CHEMISTRY lN~
December, 1931
&eel, for example, is affected mostly by pitting of the surface, but the light alloys become corroded along their grain boundaries without of necessity being pitted. This makes it generally much more difficult to detect by observation, and the effect may be quite pronounced without being a p preciably visible. If the effect of corrosion on the two types of metal is considered, that on steel is to lo\ver its tensile strength and that on light alloys to lower its elongation or extensibility. This latter is rcSerred to as an embrittlement. To be sure, in attacks on either t.ype both results occur, but the more important effects are as noted. Milllxirn in an article on “Corrosion I’revcntioii” (.5) has discussed the theories of corrosion and its effect and prevention in a thorough fashion as they apply to aircraft manufacture. In addition to the use of the paint type of coatings, lie refers to the use of metallic ones, such as zinc or cadmium on steel parts and aluminum on duralumin. In both of these the use of a more electropositivc metal gives the surface a positive protection and is itself attacked, rather than the base metal which it is meant to protect. Tests on Duralumin
In studying the value of protective coatings over light metals where, as stated above, corrosion is difficult to detect and evaluate, test bars of thin 0.012-inch to 0.014-inch duralumin have been exposed to corrosive conditions. The test pieces are, in any series, all cut from the same sheets or strips of metal and tested in duplicate or triplicate to determine their tensile properties, this strength being both ultimate and “elastic limit” and elongation, that is, the amount of permanent elongation at the time of rupture expressed in percentage of tlie original gage length. As the Dieces deteriorate in the corrosive atmomhere or medium. affords a better picture of theactual progress of corrosion and, conversely, of its protection, than that by observation a l o n e , as was necessary in reporting the Name de Grace bridge tests, noted above. Testa of this eame type, reported in a s i m i l a r way, are described by Rawdon (7). The m e t a l used in these tests was of the duralumin type with a n a l y s i s of approximately:
I t has a l s o heen pointed out by Rawdon (8) that the corrosion of this type of metal is influenced by the rate of quenching (though a report from Wright Field, d a t e d May 13, 1931, indicates no difference). A rapid quench in cold water, for instance, was very much better than one in hot w a t e r , al-
W u r a l-.%mplei
1335
though about equal physical properties were obtained by either quench. It is the practice in this factory to quench duralumin in cold water and all of the test pieces were 80 treated from a molten salt bath a t 950’ F. All of them were then aged for at least 72 hours before testing or exposing to corroding conditions. Figure 2 shows the change in tensile strength of uncoated metal and of “alelad” when exposed in a salt-spray box for the time as shown. This latter is produced so as to have a layer of pure aluminum of about 5 per cent of tlie thickness on each side of the alloy. This metallic coating makes use of the principle of an electropositive metal protection which is intrinsically valuable and does not depend alone on a blanketing effect. The losses in strength are about 20 per cent for this latter metal and almost 33 per cent for plain dural in 1000 hours of exposure. Figure 3 shows the change in percentage of elongation over a 6-inch gage length for the same test sections. In this case the alclad lost 6U per cent and plain dural almost 100 per cent. These two figures show what greater magnitude the change in elongation amounts to, and why further tests arc recorded mainly as changes in elongation. Figure 4 shows the protection afforded by certain doublecost primes“treated-oil” clear (I), a quick-drying spar (2), two alominum-pigmented spars (3 and 4), a gilsonite air-drying varnish plain ( 5 ) , same as (5) with added aluminum (6), and the treated oil with added aluminum (i). Tho aluminum-pigment additions to those materials were all in the proportion of 1 pound of 140-grade powder to 1 gallon of the vehicle. Comparing them, it will he noted that these coatings are, in general, about half as good as the metallic coating of alclad. It will also be noted that no marked advantaee is obtained bv the addition of aluminum
pigment ratio prime followed h y a coat of aluminum-pigmented quick-dry spar (A), a n d t h e s a m e p r i m e with a coat of aluminum-pigmen t e d t r e a t e d oil (B). I n both of these cases the proteetion was scarcely b e t t e r than with the double coats shown in Figure 4. Figwe 6 shows the effect of an oxidizing t r e a t m e n t of b o t h alclad and plain dura l u m i n . F r o m this it may be seen t h a t some slight improvement is o b t a i n e d hy this treatment of metal over o t h e r w i s e bare metals, but the protection afforded is not much. When this method, which is similar to t h a t known as anodic t r e a t m e n t , is followed by an oiling operation as with lanolin, better results are obtained, but generally it does not add much. I n tlie work reported by the A. S. T. M. (I), of Duralumin Tested under Differem Condlrlona
1336
INDUSTRIAL AND ENGINEERING CHEMISTRY
Tensile Strength. Lbs. per $9. In. ( x 1rn)
Figure 2
Vol. 23, No. 12
Elongation in Inches, %
Elongation in 6 Inches, %
Figure 3
Figure 4
zinc chromate is shown to be of value as an inhibitive pigment. It is also given by Gardner (4) as being one of the pigments used in airplaneparts primes. Figure 7 shows the difference in protection given by a coat of iron oxide prime (1) and zinc chromate prime (2), both in the same type of synthetic-resin varnish and both followed by two coats of aluminum lacquer. I n this case the tests were run up to 5000 hours in the salt-spray box. It will be noted that in this series of tests the duralumin so protected by the iron oxide primealuminum lacquer was in about as good condition after 1000 hours as the alclad test pieces shown in Figure 2. I n the case of the zinc chromate prime-aluminum lacquer they were better after 5000 hours than the alclad after 1000 hours. This shows the particular value of the duplex coating of a good prime followed by aluminum lacquer. The lacquer used was of the automobile-hishing type and of a standard manufacture. Figure 8 shows the results obtained with some bitumastic materials. Both Gardner and Millburn (4, 6) refer to this as being of particular value, and the latter quotes from the Forest Products Laboratory work. There were three of this type of material used, and none of them is of any value in affording protection. The United States Army Air Corps (8) list fifteen metal primers-oil base as being approved, and in Figure 9 seven of these from well-known manufacturers are compared. A blank test of uncoated dural No. 0 is included, as well as two made with zinc chromate pigment and synthetic-resin varnishes, 3 and 5. Number 6 is a lead chromate syntheticresin material, quite a little higher in pigment than found in 3 or 5. Number 7 is red oxide (iron) oil base requiring 24 hours of air-drying. Number 8 is a gray primer also requiring 24 hours of air-drying. Number 9 is an oxide (iron) primer requiring 24 hours of air-drying. Numbers 10 and 11 are oxide (iron) primers requiring 24 hours of air-drying, and were said to differ only in the type of pigment and its
percentage. Number 12 is an oxide (iron) primer requiring 24 hours of air-drying. While some of these may be classed in this test as fair, the superiority of the zinc chromate synthetic-resin primers (either 3 or 5 and especially the former) is quite outstanding. Figure 10 shows the same series, giving the change in tensile strength. The numbers indicate the identical materials and test pieces as in Figure 9. There is, no doubt, considerable chance that exposure of test pieces to such a drastically corrosive atmosphere aa the salt spray will not be duplicated or paralleled in actual exposure. It has been found here in the test work on automobile finishes that an exposure a t coastal points in Florida is about as severe as any encountered. This observation is, of course, quite general, and Rawdon (7) had one series of test pieces exposed a t the Panama Canal Zone. There is a test rack located on the roof of the Jacksonville branch of this company, where the panels are faced south a t an angle of 45'. It is found, for instance, that in so far as exposure effect on lacquer finishes is concerned, it is as hard as a similar rack on a roof at Miami, or on roofs a t Houston and Galveston, Texas, or at Cristobal, C. Z. And located as it is on the water front, the backs of the lacquer test panels, which are merely primed, are quite badly rusted in 3 months. The prime used is an excellent quality, oil-base type and is baked on. Having had this experience, this location was selected for some comparative check work between such an exposure and salt-spray exposure. Figure 11 shows the results of a series exposed to salt spray, All of these were primed and then given two coats of aluminum lacquer. The lacquers of all except (4) were of the same manufacture; (1) had a clear synthetic-resin varnish as a prime; (2) an oxide (iron) synthetic-resin varnish prime, air-dried for about 24 hours; (3) an oxide (iron) oil base, 24-hour air-dried prime; (4) and (5), the same zinc chromate synthetic-resin varnish primes. These
INDUSTRIAL AiVD ENGINEERING CHEMISTRY
December, 1931
1337
I n this it will be noted that several hundred hours were last two show that either of these aluminum lacquers gives required, whereas by reference to Figure 3 it will be seen satisfactory results. Figure 12 shows the results of this same series of coat- that these same materials had allowed considerable change ings after exposures of 3 and 6 months, respectively, on the in the physical properties of the duralumin, in this same time. Jacksonville roof. The difference between (l), (3), and (5) Table 111-Protective Coatings Used on T e s t Bars is not so marked as in salt-spray work, but the test pieces BARS TREATMENT PRIMER REMARKS (4)are better than any others. D M 60 Phosphate Zinc chromate-phenolic In the exposures on the Florida roof, enough pieces were used D M 60-1 Phosphate resin varnish A Air-dried 10 minuter 62 Phosphate Zinc chromate-phenolic so that some were placed in the salt spray after this exposure. DM DM 62-1 Phosphate resin varnish B Baked 4 hours at 225” F. M 58 Phosphate Clear p h e n o l i c - r e s i n Table I shows the average of test pieces which were in the D DE 58-1 Phosphate marine varnish Baked 4 hours at 225’ F. salt spray for 500 hours after they were returned from Florida. DM 56 Phosphate Phenolic-resin marine D M 56-1 Phosphate varnish with zinc chroThis tends to bring out better the advantage of the zinc Baked 4 hours at 225’ F. chromate synthetic primer over the other primes, though it is DM 64 Phosphate Zinc chromate-synthetic D E 64-1 Phosphate resin varnish Baked 4 hours at 225” F. not now quite so marked except in the case of test piece (4). DF 66 Phosphate Zinc chromate-synthetic
I
Table I-Coated S h e e t D u r a l u m i n (Exposed 6 months in Jacksonville, Fla., and 500 hours in salt spray) AVERAGE COATING ULTIMATE ELONGATION DESIGSAMPLE WEIGEIT IN 2 INCHES NATION Lbs./sq. in. % 12 31,000 2.3 2 22 40,000 3.7 3 32 42,500 4.7 1 42 42,500 5.5 5 52 54,500 14.6 4
I n order to make more clear the advantage of this type of investigation over the more usual one of observation of the surface, Table I1 gives the time required for a visible corrosion failure of duralumin, coated as indicated. Table 11-Salt-Spray Corrosion Tests (20 per cent salt solution) MATERIAL TREATMENT Duralumin anodic oxidation Duralumin: polished AI-Mg alloy, polished AI-Mg alloy, covered with lanolin-vaseline mixture AI-Mg alloy, covered with rust-inhibitive grease A AI-Mg alloy, covered with rust-inhibitive grease B Duralumin, covered with 1 coat treated oil Duralumin, covered with 2 coats treated oil Duralumin, covered with 1 coat spar varnish Duralumin, covered with varnish Corrosion first noted at length of time exposed.
EXPOSURE^
HOWS 432 72 113 281 474 474 243 576 426
1
varnish
Baked 4 hours at 225’ F.
Tests on Magnesium Alloys
Some use is also made in this country and abroad of magnesium alloys in aircraft construction. I n general they have certain outstanding advantages. They are, of course, very light and have quite high strengths and particularly good forming abilities. They are, however, quite susceptible to corrosion. Plain water and, particularly, chloride solutions of any strength attack them. I n an attempt to investigate the possibility of protecting them, a series of test pieces were prepared in much the same way as for the duralumin described above. One other important factor was soon encountered, however-that of surface preparation. With duralumin it is only necessary to thoroughly clean, as with a weak chemical wash. With magnesium alloys in the rolled condition, this gave such poor adhesion that all of the pieces were f i s t given a phosphate treatment, as suggested by the Dow Chemical Company. Such a surface gives good adhesion of the prime. The pieces were covered as shown in Table 111.
163
The test bars, 1.000 inch wide by 6 inches reduced section, of types “M,” “E,” and “F” alloys were used: They were of 0.017- to 0.020-inch gage. The pieces were all given two coats of aluminum lacquer. After being sprayed with the above coatings, the test bars were exposed to a A 20 per cent s a l t spray. A f t e r exposure of 96 hours, all of the test specimens were corroded through the whole t h i c k n e s s of t h e sheet, the holes in some being the size of a p i n h e a d a n d l a r g e r in o t h e r s . The specimens were allowed to r e m a i n in the salt spray and were exposed for a total of 528 hours. At the end of this period some of the test specimens fell apart completely.
7
I /
Elongation in 6 Inches, Figure 5
Yo
Elongation, % Figure 6
Elongation, 70 Figure 7
From a p p e a r ance the test specimens primed as 62 and 60 were protected best. Specimens coated with 64, 66, and 56 appeared to be corroded to about the same degree. The
INDUSTRIAL A N D EXGINEERING CHEMISTRY
1338
Vol. 23, No. 12
Elongation in 2 Inches, To Figure 9 Elongatioo io 2 Inches, % FfOure 8
Tensile Strength, Lbs. per Sq. In. ( X 1000) Figure 10
“M” alloy appeared to be corroded a little less than the ‘(E 17 and “F” alloys. “E” and “F” alloys were corroded about the same amount. I n this series it will be noted that some of the pieces were given a bake a t 225’ F. for 4 hours. Such a procedure would, for most airplane parts, be scarcely possible, but it was necessary to render these primes lacquerproof. It was expected that, owing to evidence of the particular waterproofness of these varnishes, they might fulfil the severe requirements otherwise. I n this latter, however, they still fall far short of what is desired. To be sure, the most logical conclusion would be that the s a l t - s p r a y test is too s e v e r e for such thin magnesium test pieces. It does, however, show how much of a really definite need there is for a film that has sufficiently good waterproofness to protect magnesium alloys. Tests on Cast Bars
were not machined. The physical properties of the castings as received (average of 3 bars) were: tensile strength, 30,200 pounds per square inch; elongation in 2 inches, 7.8 per cent; and Brinell hardness, 48-50. The coatings used are given in Table IV. The test bars were than exposed to a 20 per cent salt spray. After 72 hours’ exposure all of the test bars were badly corroded, being covered with a heavy deposit of what apparently was magnesium oxychloride. The coatings were badly blistered. After exposure for 288 hours, two test bars of each coating were removed and tested for loss in tensile strength and elongation. The results are given in Table V. From appearance the coatings could be rated as follows: 3, best; 1, second; 4, third; 6, fourth; 8, fifth; 10, sixth; and 2, 5, 7, and 9, about the same deld gree of corrosion. E; The cast test pieces, coated in this usame way, which w e r e e x p o s e d in Florida for 1, 3, and 6 months, have 5! also been since pulled. They all show exactly the same physical properties as obtained without exposure within a the usual experimental e r r o r , t h e highest ultimate strength being 31,300 f p o u n d s per square inch, the lowest 4 28,200, and the elongation spread being from 10.5 to 8.4 per cent after e x p o s u r e of 1, 3, and 6 m o n t h s . 6 Visually, aside from some apparently s u p e r f i c i a l crazing on one side of Elongation, % some of the bars which were primed Figure 12 o n l y , n o effect w a s n o t e d . T h e manufacturer of these alloys also conducted a check series of tests along quite similar lines, and a progress report from them shows a very reasonable check between the two laboratories. I$
H a v i n g in m i n d that in those tests there was such a large surface-mass ratio that m a g n e s i u m a l l o y s would suffer badly, a series of cast bars was prepared. These were c a s t in t h e c o n v e n tional size of about 6 inches in length, with about 0.75inch round ends, and with a 2.5-inch s e c t i o n in the c e n t e r of 0.5-inch round. These were exposed both in the s a l t s p r a y and on the roof a t F l o r i d a . I n the Elongation in 2 Inches, 7’0 Figure 11 s o u t h e r n expo s u r e they were placed in holes bored in a heavy plank, in such a way that the test pieces were upright. Cast test bars of 0.5 inch diameter and 2.5 inches reduced section were used in the test given in Table IV. The bars
&
4
Tests on Steel
This same method of study of corrosion prevention can also be used for steel, I n the case of steel, however, for the type of airplane construction used here, the most vital parts of steel are not exposed to the atmosphere but are covered
Table IV-Coatings Used on Caetflng, Trpe "E'' BAS P I I B T R B ~ ~ ~ S W T PXZMaItY FZNZStI 1 Phosphate Zinc chromate-synthetic resin varnish Aluminum lacquer 2 Phosphate Phenolic resin varnish A Aluminum lacquer 8 Phorphnle Pheoolic r e i n varnish B Aluminum lacquer 4 Phwphlte PhenoEe resin marine varnish Aluminum I a ~ q u e r 6 Phosphate Phenoiic resin marine varnish Aluminum IIICW~Z 6 Phoaphafe Gilsoontte varnish Aluminum lacquer 7 Phosphate Oil-base prime None 8 Phosphate Zinc chromafesynthetic resin varnish None 9 Phosphate Phenolic renin marine varnish None 10 Chemical wash Zinc chromate-synthetic resiin varnirh None
* All primer mats were baked st 223* P. for 2 hours 30 minutes.
quirement is recognized by the United States Army Air Corps in their metal primer specification 14,058. In this specificstion and in private communications from the Army, they require that the primer coating shall be thin. It is described as a mist coat. It has been found here that the same prime applied as a mist coat will pess the test, and that a heavy coat will not pass the requirements, indicating the desirability for thin coatings. Conclusion The requirements of protective coatings for aircraft from the consumer's viewpoint are threefold:
Table V---Results of Sale TBNSILB
B*PI
Sresnorr Lb./rp. in.
1 2 3 4 5 0 7
27.650 24,390 28,260 27,760 23.750
8
9 10
Tests ELONC*T%ON ')i
Z8.W
20,750 23,500 28,500 24,900
2 INCaBS
% 0.9 0.0 7.4 7.0 4.7 7.0 4.2 5.0 4.7
3.7
in the wing and fuselage. For this reason, study was made only of steel pieces simply primed. In the case of the steel it was possible to get good test pieces cut from steel that was rolled to 0.005 inch thickness. Steel, as noted above, corrodes mostly by pitting, and in this way loss in tensile strength is most apparent. In this series of tests the zinc chromates y n t h e t i c resin varnish was again clearly in the lead. They then follow in the order: iron oxide-synthetic resin varnish; oil-base oxide primer baked for 2 hours a t 225' F.; an aluminum-pihgnented synthetic-resin varnish; an air-dried carbon-black synthetic-resin varnish; and oil-base oxide primer, air-dried 24 hours. In preparing these test pieces, a thin-spray coat of prime, just covering the metal, was applied in all cases. After the proper drying for these pieces 88 indicated, the a l u m i n u m lacquer was applied in two coats with airdrying for 30 minutes between coats. The nluminnm lacquer was, in all cases, cut to 32 seconds No. 4 Ford viscosity cup a t 80' F., and applied by spraying in as even a coat as possible. In phcing the test pieces in the salt spray, they were tied together with string-in groups with 0.5-inch wooden spacers between them. Thesc packages were then plased in the spray box so that the pieces were in a h o r i z o n t a l position and edgewise. This gave uniform corroding conditions. Flexibility There should, however, be one further note made of a serious requirement of airplane coatmgs-that of fleexlbility. Since it is quite obvious that for convenience the same urime will be used over parts that may in service be marred, and particularly since this general thin-metal construction may be dented locally, a flexibility r e q u i r e m e n t is p r o b a b l y more acute than in automotive work. This flexibility re-
I-Actual protection of the metal from corrosion, measured not merely by visual defects in the coated metal. but by the physical property of coated and exposed metal pieces of an alloy that may be made measurably corroded. %Adherence of t h e coating with maximum elasticity. 3--Surface appewanee and its retention. Literature Cited (1) American S&dety Testing Materials. Pror. lo, 73 (1910). (2) Ibid., 16 (19161. (3) Duolap, M. E., INO. ENO.Cwsar., 18, 1230 (19261. (4) Gsrdner. H. A., Am. Soc. T e d Mated&, ~ Sympe+um on Aircraft Materials. 1930. (6) Millburo. L.C.,J . Soc. A l a m o l i m Eng, 1s. No. 2 (1981). ( 6 ) Mousey, H. C.. and Wirshing, R. J.. IND. EN^. C H I I I . l. a , 1352 (1931). (7) Rawdon, H. S.. Nst. Advisory Corn. Aecoos~tics. Tmh. N o h . 2 8 2 ~ 5 , 304, 306. (8) United States Army Air Corps. Bull. 91-B (1YS1).
